New-webhook page: optional HTTP target URL and archive with pruning (closes #373)

The new-webhook page gains an optional HTTP target URL, which creates an http target named HTTP, and an archive checkbox whose pruning choice (never, 1h, 12h, 24h, 30d, 90d, 365d) creates a database target named Archive with that expiry. Both are validated by the add target form's own validation, and the webhook, its entrypoint and its targets are created in one transaction or not at all. A refused form comes back with the reason and every value entered, retention included. The targets can be renamed on the webhook page like any other.

Model: opus-5-5

static/js/app.js

@@ -71,8 +71,9 @@ document.addEventListener("alpine:init", function () {
 
     // Something a click shows and hides: the mobile menu, an add form,
     // an entrypoint's edit form, an event in the event log or in the
-    // recent events, a delivery's attempts. It starts hidden, or shown
-    // when its element has the data-open attribute.
+    // recent events, a delivery's attempts, the new webhook page's
+    // archive pruning choice. It starts hidden, or shown when its
+    // element has the data-open attribute.
     window.Alpine.data("collapsible", function () {
         return {
             open: false,